ECLOUD Simulations for CESR Witness Bunch Tune Shift Measurements Jim Crittenden Cornell Laboratory for Accelerator-Based Sciences and Education.

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Presentation transcript:

ECLOUD Simulations for CESR Witness Bunch Tune Shift Measurements Jim Crittenden Cornell Laboratory for Accelerator-Based Sciences and Education

2 9 July 2008 ILCDR08/ J.A.Crittenden CESR Tune Measurements 2007 Train of ten 1.9 GeV, 0.75 mA positron bunches generates the electron cloud Measure tune shift and beamsize for witness bunches at various spacings using bunch-by-bunch, turn-by-turn beam position monitor Error bars represent measurement spread CesrTA Electron Cloud R&A Overview Mark Palmer, 5 March 2008, TILC08, Sendai, Japan Strong vertical focusing effect Horizontal tune shift much smaller, either sign Positron Beam Vertical defocusing effect Horizontal tune shift much smaller Large spread in horizontal tune measurements for witness bunches Electron Beam

3 9 July 2008 ILCDR08/ J.A.Crittenden ECLOUD program V3.2 (G. Rumolo & F. Zimmerman) –2D, intensive development , flexible, 20k lines readable F77 –Adapted to CESR tune shift measurements 2007 and CesrTA North Area Triple-RFA measurements –Continued development (output info, graphics, field calculations) –Talks/results archive POSINST (Gerry Dugan with Miguel Furman) CLOUDLAND (Joe Calvey with Lanfa Wang) Electron Cloud Calculations Underway at Cornell An ambitious effort comparing simulations to each other and to measurement is now underway at Cornell See ILCDR08 talks by G. Dugan and J.Calvey

4 9 July 2008 ILCDR08/ J.A.Crittenden Interpretation by K.Ohmi, et al. Electron Cloud Effects at CESR and KEKB K. Ohmi, KEK 29 February 2008, Cornell seminar 2nd order moment ( c, c ) of electron cloud distribution gives tune shift, where c = > 2 Beam and Photoelectron interactions in positron storage rings Phys. Rev. Lett. 75 (1995) 1526 Study of Coherent Tune Shift Caused by Electron Cloud in Positron Storage Rings, APAC'01, WEP056 ● Bunch population N p =2x10 10 ● electrons created by a bunch passage in a meter ● N p xY pe =1.7x10 9 (5GeV) 6.8x10 8 (2GeV) ● If electrons are accumulated 5 times, ● electron line density (m -1 ) 8.5x10 9 (5GeV), 3.4x10 9 (2GeV) ● volume density(m -3 ) 1.7x10 12, 6.8x10 11 ● Corresponding Tune shift , (7.4e-4/bunch) ● Beam line density N p /4.2=4.8x10 9 if  x ~  y where ➢ The calculation depends on ring- averaged quantities ➢ The observed tune shifts are quantitatively consistent with the expected average cloud density ➢ The charge density distribution and the resulting electric field can be quite complicated ➢ Don't forget about the beampipe !

5 9 July 2008 ILCDR08/ J.A.Crittenden Dave Rubin's Suggestion In mid-February, Dave Rubin suggested investigating the cloud spacecharge field calculation used by the ECLOUD program. Systematics of field calculation Precision of numerical approximations Field gradients' dependence on cloud simulation parameters This suggestion resulted in an extraordinarily fruitful line of investigation E X (V/m) E Y (V/m) DLR: For a ring-averaged beta value of 30 m, an average field gradient of 3600 V/m 2 will yield a tune shift of 1 kHz.

6 9 July 2008 ILCDR08/ J.A.Crittenden CESR Tune Measurements Positron Beam Input Parameter Set Ten 0.75 mA bunches 1.9 GeV 30 empty witness bunches No magnetic field Elliptical chamber 4.5x2.5 cm 0.1 s.r. photon per beam particle per m 20% reflected s.r. photons 10% photoelectron per s.r. photon Peak secondary yield of 2.0 Peak SEY energy of 310 eV ➢ Predicted vertical tune shift 50% lower than measured even if the entire ring were B-field-free ➢ Impressively similar time structure ➢ Vertical gradient factor 5 higher ➢ Horizontal gradient bipolar Positron Beam Field gradients averaged over a 9 mm x 5 mm region at the center of the beam pipe dE Y /dY (V/m 2 ) dE X /dX (V/m 2 )

7 9 July 2008 ILCDR08/ J.A.Crittenden CESR Tune Measurements Electron Beam Electron Beam ➢ Large spread in horizontal tune measurement for the witness bunches not presently understood ➢ Vertical gradient factor 3 lower than for positron beam, as measured ➢ Vertical gradient continues to grow after passage of filled bunches ➢ Polarity of vertical tune shift and gradient as expected for electric field effect ➢ Calculated field gradient shows repulsive effect of beam dE X /dX (V/m 2 ) dE Y /dY (V/m 2 )

8 9 July 2008 ILCDR08/ J.A.Crittenden ECLOUD Charge Buildup Positron Beam Beam-pipe-averaged cloud density (m -3 ) Average cloud density lower by 30% for electrons while field gradients are a factor of 3 smaller ! Electron Beam

9 9 July 2008 ILCDR08/ J.A.Crittenden Cloud Density and Field Gradients Cloud profile after last filled bunch Positron Beam No cut on energy E kin > 10 eV Large gradients occur also when the central cloud density is small. These conditions are such an example. Another example is the electrostatic repulsion of an electron beam.

10 9 July 2008 ILCDR08/ J.A.Crittenden Critical Parameters e + B=0 e + B ⊥ = 800 G e + B ∥ = 20 G e + B=0, v.c. image charges off e + B=0, round v.c., ρ =4.5 cm e + B=0, REFL=0% e + B=0, REFL=100% e + B=0, v.c. 5.5 x 2.5 cm e - B=0 e - B=0, REFL=100% e - B ⊥ = 800 G 1.6E E E E E E E E E E <5 Input Parameter change dE Y /dY (V/m 2 ) Beam-pipe-averaged density (m -3 ) dE X /dX (V/m 2 ) NB: The measured vertical tune shift corresponds to a field gradient of 3000 V/m 2 continuous around the ring with an average beta value of 30 m. Vertical gradient reduced factor 10 e + Baseline for comparison e - Baseline for comparison Vertical gradient reduced factor 40 !! Small effect of BP conductivity BP shape effect on vertical/horizontal Flat azimuthal source distribution Beampipe shape more important than azimuthal source distribution Wider horizontal beampipe ellipse Dipole field reduces gradients

11 9 July 2008 ILCDR08/ J.A.Crittenden Near -Term Plans Continue investigation of simulation input parameters Compute field gradients averaged over beam profile Calculate ring-averaged field gradients & tunes Continue beam-pipe shape study All suggestions welcome Continue investigation of simulation input parameters Compute field gradients averaged over beam profile Calculate ring-averaged field gradients & tunes Continue beam-pipe shape study All suggestions welcome

12 9 July 2008 ILCDR08/ J.A.Crittenden Second Witness Bunch ? Input Parameter Set Ten 0.75 mA bunches 1.9 GeV 6 empty witness bunches No magnetic field Elliptical chamber 4.5x2.5 cm 0.1 s.r. photon per beam particle per m 20% reflected s.r. photons 10% photoelectron per s.r. photon Peak secondary yield of 1.6 Peak SEY energy of 170 eV ➢ Need to investigate dependence on input parameters ➢ Dip at second witness bunch seen Positron Beam Field gradients averaged over a 9 mm x 5 mm region at the center of the beam pipe dE Y /dY (V/m 2 )dE X /dX (V/m 2 )